readLock.lock();
- This means that if any other thread is writing (i.e. holds a
write lock) then stop here until no other thread is writing.
- Once the lock is granted no other thread will be allowed to write
(i.e. take a write lock) until the lock is released.
writeLock.lock();
- This means that if any other thread is reading or writing, stop
here and wait until no other thread is reading or writing.
- Once the lock is granted, no other thread will be allowed to read
or write (i.e. take a read or write lock) until the lock is released.
Combining these you can arrange for only one thread at a time to have write access, but as many readers as you like can read at the same time except when a thread is writing.
Put another way. Every time you want to read from the structure, take a read lock. Every time you want to write, take a write lock. This way whenever a write happens no-one is reading (you can imagine you have exclusive access), but there can be many readers reading at the same time so long as no-one is writing.